[I]t could mean big changes for large portions of the armed forces, including the Marines, which had selectively banned social media on work computers.

The Department of Defense also had bans in place since 2007 on accessing certain bandwidth-gobbling Web sites like YouTube on its network. . . .

Defense Secretary Robert Gates, 66, has said that he wants to use social networking to help the Pentagon interact with U.S. military members, many of whom are in their early 20s.

But opponents have cited the risks of information leaks, of opening gateways to hackers, along with a potential overload of precious bandwidth on the Defense Department’s network.

The new policy says commanders will still need to defend against cyber-attacks and block access to online pornography, gambling and sites promoting “hate-crime related activities.”

It also allows commanders to temporarily limit Internet access if the bandwidth is overwhelmed, a key caveat for U.S. forces fighting the wars in Iraq or Afghanistan, or stationed in remote, rugged places around the globe.